This invention relates generally to RF power transistors, and more particularly the invention relates to providing a low parasitic impedance input/output feed structure in a distributed power transistor.
Power transistors (MESFET, MOSFET, BIPOLAR, HBT) for RF and Microwave frequencies are made of multiple building blocks or individual transistors which are driven in parallel with the outputs of the transistors combined as the power output. A commonly used drive structure is the xe2x80x9cdivide-by-twoxe2x80x9d feed, which is used over and again as shown in FIG. 1. In FIG. 1, the input signal is applied at terminal 10 with the feed path 12 divided by 2 as many times as necessary to drive the individual transistors 14. The xe2x80x9cdivide-by-twoxe2x80x9d pattern can be used several times as long as each path has equal input length to the individual transistors. The input terminals of transistors 14 can be interconnected also by line 15. The output side is xe2x80x9cpower combinedxe2x80x9d by the same structure in reverse order.
The high frequency response of a transistor is affected strongly by the input capacitance to the transistor, which is the gate to source capacitance in a field effect transistor and a base-to-emitter capacitance in BIPOLAR and HBT transistors. Therefore, the standard rule of thumb has been to avoid the overlap of the feed structure to any source or emitter electrode connection and thereby minimize the input capacitance.
The feed structure represents a large inductance in a high power transistor with the input inductance adversely effecting the input matching circuit bandwidth. Also, an xe2x80x9codd modexe2x80x9d oscillation can occur with a large input and output impedance among the building block transistors as illustrated in FIG. 2. As shown in FIG. 2, the input at contact 10 is applied through inductors 12 to the base of transistors 14, the output is taken at the collectors and applied through inductors 18 to the output terminal 20. As illustrated by the dotted signal configuration 22, the inductive feed can cause an oscillation with the input signal at transistors 14 being 180xc2x0 out of phase thereby canceling the signal of the transistors at input 10 and output 20.
The present invention is directed to an improved low inductance power feed structure and in which possible xe2x80x9codd modexe2x80x9d operation is reduced.
In accordance with the invention, the input/output feed structure to the plurality of transistor elements in an RF power transistor is placed in close proximity to a ground plane whereby the feed structure functions as a low impedance microstrip line in feeding each of the transistor elements. The ground plane is connected through vias, for example, to a backside ground plane on the bottom of the transistor semiconductor chip.
More particularly, an RF power transistor includes a semiconductor body having a plurality of individual transistors therein, a first ground plane on a bottom surface of the semiconductor body and a second ground plane on a top surface of the semiconductor body which is electrically connected to the first ground plane and to which common elements of individual transistors are connected. A dielectric insulating layer is provided over the second ground plane, and a feed structure is then formed on the insulating layer and overlying the second ground plane, the feed structure preferably comprising a metal layer which has been selectively patterned to form the feedlines to the individual transistors.
The semiconductor body can comprise silicon or a III-IV semiconductor material such as gallium arsenide or indium phosphide. The insulating layer is preferably silicon oxide, silicon nitride, polyimide, or a combination thereof. The individual transistor elements can be a MESFET, MOSFET, BIPOLAR or HETROJUNCTION BIPOLAR transistor (HBT).
Not only is the parasitic impedance provided by the microstrip line reduced, but also total transistor size can be increased before the xe2x80x9codd mode oscillationxe2x80x9d occurs.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.